U.S. Pat. Nos. 3,317,322 and 3,761,276 disclose that when chemical sensitization is applied to the surface of internal latent image-type silver halide grains comprising a core of silver halide doped with metal ions, or chemically sensitized, or subjected to both treatments, and a shell of the silver halide covering at least light-sensitive sites of the core (hereinafter referred to as "core/shell type grains"), a reversal image is obtained by development in the presence of foggants, or by a direct reversal process of the type that an overall light-exposure is applied at the time of development.
These core/shell type grains, the core of silver halide being at least chemically sensitized, when employed in a fine grain silver halide emulsion with a mean grain size of not more than 0.4 .mu.m in order to obtain a direct positive photographic material which provides good graininess and high resolution power have the disadvantages that only a reversal image in which D.sub.max is low and D.sub.min is high is obtained, and that the light-sensitive material has insufficient stability with time.
That is, even if an internal latent image-type silver halide emulsion is prepared by applying chemical sensitization to the core of silver halide grains with a mean grain size of not more than 0.4 .mu.m to the extent that when applied to the core of silver halide grains with a relatively large grain size (e.g., more than 0.4 .mu.m) (for example, to the extent applied in preparation method of Emulsion A at Example 1 in U.S. Pat. No. 3,761,276) or applying chemical sensitization to the core of silver halide grains with a mean grain size of not more than 0.4 .mu.m to such an extent that addition amount of chemical sensitizer is increased in proportion to the increase of specific surface area of the core of silver halide grains (i.e., in proportion to becoming small in the core of silver halide grains) and then applying chemical sensitization to the shell of silver halide grains, a satisfactory reversal image cannot be obtained, a satisfactory image cannot be formed; D.sub.max is low or D.sub.min is high and good reversal performance cannot be obtained.